A well known type of data communication system uses a single electrical conductor, usually a coaxial cable, to connect a data terminal to a controller capable of supporting many terminals. The most common form of this system is a half-duplex system. A half-duplex system is one in which data can be transmitted only in one direction on the cable at any given time; that is, data cannot be transmitted in opposite directions on the cable at the same time.
Conventionally, a controller transmits data to a terminal and waits for a response, indicating the terminal has received the data. If no response is received within a certain period of time, the controller assumes the data was not received and takes an appropriate action. The controller may, for example, re-transmit the data or may send an error message to a system operator.
Since the controller initiates all communications with the terminal, only the controller or the terminal should transmit at any given time. There should be no collision on the cable between controller-transmitted data and terminal-transmitted data.
Because the controller initiates data transfers on an as-needed basis, there will be times during which the half-duplex line is inactive or idle. Receiver circuitry in either the controller or the terminal may enter an idle state during such idle periods. To re-initialize the receiver circuitry, a preamble (a series of signals which precede actual data) is conventionally employed. The preamble resets a receive clock and delineates the beginning of the data itself. A postamble, a second series of signals which follow the actual data, is used to delineate the end of the message.
In some situations, it is desirable to connect a half-duplex system of the type described to a simplex system. A simplex system is one that uses separate conductors to carry transmitted and received data. Usually, data can be transmitted and received simultaneouly over the separate conductors.
One problem with any half-duplex/simplex interface is that the simplex transmitter circuit and the simplex receiver circuit have a common electrical connection at the half-duplex line connector. Means must be provided for preventing received simplex signals from returning or "echoing" to the simplex transmitter circuit through this common electrical connection.
Where the half-duplex and simplex systems use the same media, there are components which can be used to prevent such echoing from occurring. In systems using electrical conductors, devices known as hybrid transformers permit received simplex data to be applied to the half-duplex cable but not the local simplex transmitter. Where optical fibers are used, beam splitters can be used to transmit data to the half-duplex fiber but not to the local simplex optical transmitter.
Neither of these solutions can be used in a hybrid system; that is, an electrical half-duplex system interfaced to an optical simplex system.
A hybrid environment may exist where a terminal controller designed for electrical half-duplex operation is attached to a high-speed optical bus having dual transmit and receive optical fibers. The optical bus may be used either as a serial channel to a remote host processor or as a secure, high speed link to a supported terminal.
U.S. Pat. No. 4,288,869, assigned to the assignee of the present invention, addresses the problem of echoing in an electrical half-duplex/optical simplex interface. According to this patent, received simplex signals are applied to three parallel paths, each of which delays the signals by a different amount. The half-duplex system is connected to the path which introduces the intermediate amount of delay. The three paths provide inputs to a single NAND gate, the output of which is applied to a driver for the simplex transmitting fiber. The introduction of different amounts of delay into the received simplex signal prevents that signal from being passed through to the simplex transmitter.
The solution proposed by the patent works well for optical systems in which optical receivers operate properly for light levels which remain nominally constant over significant periods of time. Unfortunately, such receivers typically display low sensitivity to changes in applied optical signals and can not be used effectively with long optical busses.
A sensitive optical receiver will emit noise signals when a nominally constant optical signal is received for a lengthy period of time. The noise may interfere with the proper functioning of the circuit shown in the referenced patent. Moreover, over even relatively short periods of time, the internal sampling levels in a sensitive receiver may be so disturbed that subsequent optical bits are not correctly translated to electrical bits.
To avoid the problems mentioned above, simplex systems with sensitive optical receivers send a continuous idle signal (for example, repeated "1"'s) when data is not being transmitted. Such idle signals are conventionally referred to as keep-alive signals.
In a hybrid system, a continuous idle signal can cause problems since idle signals flowing in one direction would be collide with data flowing in the other direction.